In the normal human heart, the sinus node, generally located near the junction of the superior vena cava and the right atrium, constitutes the primary natural pacemaker initiating rhythmic electrical excitation of the heart chambers. The cardiac impulse arising from the sinus node is transmitted to the two atrial chambers, causing a depolarization known as a P-wave and the resulting atrial chamber contractions. The excitation pulse is further transmitted to and through the ventricles via the atrioventricular (A-V) node and a ventricular conduction system causing a depolarization known as an R-wave and the resulting ventricular chamber contractions.
Disruption of this natural pacemaking and conduction system as a result of aging or disease can be successfully treated by artificial cardiac pacing using implantable cardiac pacing devices, including pacemakers and implantable defibrillators, which deliver rhythmic electrical pulses or anti-arrhythmia therapies to the heart at a desired energy and rate. One or more heart chambers may be electrically paced depending on the location and severity of the conduction disorder.
Modern pacemakers and implantable defibrillators possess numerous operating parameters, such as pacing pulse energy, pacing rate, sensing threshold, pacing mode, etc., that must be programmed by the clinician to satisfy individual patient need. In practice, this programming process can be time consuming and complicated. A common goal of pacemaker manufacturers, therefore, is to fully automate pacemaker function in order to minimize the complexity of programming operations and to maximize the safety and effectiveness of the cardiac pacing device.
One function of the pacemaker is to deliver a pacing pulse of sufficient energy to depolarize the cardiac tissue causing a contraction, a condition commonly known as “capture.” Automating this function continues to be a strong focus of development efforts by pacemaker manufacturers. One approach to ensure capture is to deliver a fixed high-energy pacing pulse. While this approach, used in early pacemakers, is straightforward, it quickly depletes battery charge and can result in patient discomfort due to extraneous stimulation of surrounding skeletal muscle tissue.
Therefore, the aim commonly strived for in the pacemaker industry is to deliver pacing pulses at or slightly higher than the capture “threshold.” Threshold is defined as the lowest stimulation pulse energy at which capture occurs. By stimulating the heart chambers at or just above threshold, comfortable and effective cardiac stimulation is provided without unnecessary depletion of battery charge. Capture threshold, however, is extremely variable from patient-to-patient due to variations in electrode systems used, electrode positioning, physiological and anatomical variations of the heart itself, and so on. Therefore, at the time of device implant, the threshold is determined by a clinician who observes an ECG recording while pulse energy is decreased, either by decrementing the pulse amplitude or the pulse width, until a loss of capture occurs.
Typically, the pulse width is fixed and the pulse amplitude is decremented. The stimulation pulse energy is then programmed to a setting equal to (or as a function of) the lowest pulse energy at which capture still occurred (threshold) plus some safety margin to allow for small fluctuations in threshold. Selection of this safety margin, however, can be arbitrary. Too low of a safety margin may result in loss of capture, an undesirable result for the patient. Too high of a safety margin will lead to premature battery depletion and potential patient discomfort.
Furthermore, the threshold will vary over time within a patient as, for example, fibrotic encapsulation of the electrode occurs during the first few weeks after surgery. Fluctuations may even occur over the course of a day or with changes in medical therapy or disease state. Hence, techniques for monitoring the cardiac activity following delivery of a pacing pulse have been incorporated in modern pacemakers in order to verify that capture has indeed occurred. If a loss of capture is detected by such “capture-verification” algorithms, a threshold test is performed by the cardiac stimulation device in order to re-determine the threshold and automatically adjust the stimulation pulse energy. This approach, called “automatic capture”, improves the patient's comfort, reduces the necessity of unscheduled visits to the medical practitioner, and greatly increases the pacemaker's battery life by conserving the energy used to generate stimulation pulses.
The most widely implemented technique for determining whether capture has occurred is monitoring the intracardiac electrogram (EGM) received on the implanted cardiac electrodes. Heart activity is monitored by the pacemaker by keeping track of the stimulation pulses delivered to the heart and examining the EGM signals that are manifest concurrent with depolarization or contraction of muscle tissue (myocardial tissue) of the heart. Through sampling and signal processing algorithms, the presence of an “evoked response” following a stimulation pulse is determined. The “evoked response” is the depolarization of the heart tissue in response to a stimulation pulse, in contrast to the “intrinsic response” which is the depolarization of the heart tissue in response to the heart's natural pacemaking function.
When capture occurs, the evoked response of a P-wave or a R-wave indicates contraction of the cardiac tissue in the atria or ventricles, respectively, in response to the applied stimulation pulse. For example, using such an evoked response technique, if a stimulation pulse is applied to the ventricle (hereinafter referred to as a Vpulse), a response which is sensed by ventricular sensing circuits of the pacemaker following the application of the Vpulse and meets the capture detection criteria, is presumed to be an evoked response that evidences capture of the ventricles.
However, for several reasons it can be difficult to detect a true evoked response. First, because the evoked response may be obscured by a pacing pulse and therefore difficult to detect and identify. Second, the evoked response may be difficult to distinguish from an intrinsic response since an intrinsic response may occur approximately the same time as an evoked response is expected to occur. Third, the signal sensed by the pacemaker's sensing circuitry immediately following the application of a stimulation pulse may not be a QRS complex but noise, either electrical noise caused, for example, by electromagnetic interference, myopotential noise caused by skeletal muscle contraction, or “cross-talk,” defined as signals associated with stimulation pulses or intrinsic events occurring in other heart chambers.
Another signal that interferes with the detection of an evoked response, and potentially the most difficult for which to compensate because it is usually present in varying degrees, is lead polarization. A lead-tissue interface is where an electrode of the pacemaker lead contacts the cardiac tissue. Lead polarization is commonly caused by electrochemical reactions that occur at the lead-tissue interface due to application of an electrical stimulation pulse, such as an atrial pacing pulse, at the interface.
If the evoked response is sensed through the same electrodes through which the stimulation pulses are delivered, the resulting polarization signal, also referred to herein as an “afterpotential,” formed at the electrode can corrupt the evoked response that is sensed by the sensing circuits. This undesirable situation occurs often because the polarization signal can be three or more orders of magnitude greater than the evoked response. Furthermore, the lead polarization signal is not easily characterized; it is a complex function of the lead materials, lead geometry, tissue impedance, stimulation energy and other variables, many of which are continually changing over time.
In each of the above cases, the result may be a false positive detection of an evoked response. Such an error leads to a false capture indication, which in turn leads to missed heartbeats, a highly undesirable and potentially life-threatening situation. Another problem results from a failure by the pacemaker to detect an evoked response that has actually occurred. In that case, a loss of capture is indicated when capture is in fact present, which also constitutes an undesirable situation that could cause the pacemaker to unnecessarily invoke the threshold testing function in a chamber of the heart, and to inappropriately deliver backup pulses.
Automatic threshold testing is only invoked by the pacemaker when loss of atrial or ventricular capture is detected, or a predetermined duration has expired. An exemplary conventional threshold test procedure is performed as follows. When loss of capture is detected, the pacemaker increases the stimulation pulse output level to a relatively high predetermined testing level at which capture is certain to occur, and thereafter decrements the output level until capture is lost. The stimulation energy is then set to a level slightly above the lowest output level at which capture was attained. Thus, capture verification is of utmost importance in proper determination of the pacing threshold.
It would thus be desirable to provide an implantable, multi-chamber cardiac stimulating device in which an inter-chamber conduction search, conduction time measurement, and reliable capture verification are performed. It would also be desirable to provide a system and method for capture verification that avoids the adverse effects of polarization and noise. It would further be desirable to enable the device to perform capture verification without requiring dedicated circuitry and/or special sensors.